Calculate the solubility of CaC2O4 in an aqueous solution buffered at pH 4.0.

The relevant reactions are CaC2O4=>Ca2+ + C2O4 2- Ksp

H2C2O4=>HC2O4- + H(+) Ka1

HC2O4- => C2O4(2-) + H(+) Ka2

I know how to obtain the solubility by using the mass balance equation, and I got the correct answer.

I balanced it like this: [Ca2+]=[C2O42-] + [HC2O4-] + [H2C2O4], solved for [Ca2+] and got the right answer

But however, when i used the charge balance equation: 2[Ca2+] + [H+] => 2[C2O4(2-)] + [HC2O4-] + [OH-], and solved for [Ca2+] but i didn't get the correct answer.

Can anyone tell me why I can't use the charge balance equation for this question?

Thanks